Atomic force microscopy is a technique for measuring extremely small dimensions, which takes advantage of the small forces created between the extreme end of an ultrafine tip and a surface which is to be measured or profiled. AFM may be used in the production of integrated circuit (IC) chips to measure the critical dimensions of the various components of the circuit, for example, the linewidth of the features of the IC wafer. This technique is known as AFM micrometrology and is becoming of increasing importance since as the scale of integration increases, there is a corresponding decrease in the dimensions of the features of the IC wafer which must be measured or profiled.
One type of AFM makes use of ultrafine tips which are mounted on spring-like cantilevers. As the tip is brought closer to the surface which is being profiled, the force between the surface and the tip causes the tip to deflect the cantilever. The degree of deflection may be measured to resolve the features of the surface. However, the use of this technique with conventional AFM tips to accurately profile deep structures such as trenches formed on IC wafers is limited. A conventional tip such as parabolic tip 1 shown in FIG. 1a loses resolution as the radius of the probe increases with depth, leaving a "dead zone" between the lower regions of the tip and the inner edge of the trench, which results in a loss of information about the inner edge of the trench. Conventional needle tips such as tips 3 shown in FIG. 1b provide better resolution. However, needle tips are extremely vulnerable to being damaged or broken, for example, due to crashes with the vertical edges of the trench.
In order to overcome these drawbacks in micrometrology of deep trenches, three-point tips such as tip 5 shown in FIG. 2 have been developed. Tip 5 is disposed in a trench. Such tips may be vibrated in two dimensions using piezo-electric elements, with the tip held stationary except for the vibration, and the surface to be profiled moved towards the tip. When the surface is close enough to the tip to result in creation of interactive forces, the tip vibration is damped. Damping of the vibration is therefore indicative that the surface of the trench is a certain distance from the tip at that particular location of the trench. By repeating the process at different locations, the relative depth of the trench at different locations can be determined to obtain a surface profile. This technique is discussed in Nyyssonen et at, "Two-dimensional Atomic Force Microprobe Trench Metrology System ", Journal of Vacuum Science Technology, B9(6), November/December 1991, pp. 3612-16.
In order to use this technique, tips 5 must have three points, one each for sensing the bottom surface and the right and left edges. The overall width w of the tip determines the smallest trench width which may be measured, the longitudinal distance d from the widest point of the tip to the bottom of the tip measures how close to the bottom of a trench a width can be measured, and the lateral distance D between the location of the tip at its widest extent and the location of the tip at its narrowest extent determines the limit of undercut which can be determined accurately. The smaller these dimensions can be made, the more accurate the resulting profile.
One technique for forming a three point tip is disclosed in U.S. Pat. No. 5,171,992, to Clabes et at, incorporated by reference. With reference to FIG. 3, conventional silicon probe tip 10 which serves as a substrate is manufactured in a conventional manner, for example, by known techniques for selective etching and undercutting. Such techniques are disclosed in Albrecht et al "Microfabrication of Cantilever Styli for the Atomic Force Microscope", Journal of Vacuum Science Technology, A8(4), July/August 1990, pp. 3386-3396. Thereafter, conical needle-shaped tip 18 is formed on forward surface 14 of substrate by conventional electron beam chemical vapor deposition (CVD).
In CVD, substrate 10 is disposed in an evacuated chamber of a conventional electron beam unit. An organo-metallic compound gas stream is introduced into the chamber, and electron beam 12 is applied to forward surface 14 of silicon substrate 10. Electron beam 12 causes decomposition of the gas, and preferential deposition of the decomposed products onto surface 14. As the process continues, layers of the decomposed products build up on surface 14 to create tip 18. Tip 18 includes a carbon matrix in which metal particles are dispersed, that is, tip 18 is organo-metallic.
In order to form a three-point tip, the process described above with reference to FIG. 3 is performed until shank 20 is formed on a substrate, as shown in FIG. 4a. Thereafter, beam 12 is moved sideways in successive steps from the center of shank 20 to begin lateral buildup of organometallic material and thereby form side point 20a, as shown in FIG. 4b. The geometry of side point 20a will depend upon the intensity of the electron beam, beam size, separation between successive horizontal steps, the duration of the beam at each step and the total number of steps. When the desired length of point 20a is reached, electron beam 12 is returned to the center of shank 20, and is stepped in the opposite direction to form point 20b as shown in FIG. 4c. Finally, electron beam 12 is returned to the center of the forward surface of shank 20 to form center point 20c to form a complete three point tip. FIG. 4e is a photomicrograph showing various three-point tips formed by using this prior art technique.
Though useful in forming three-point AFM tips for critical dimension measurements, use of the CVD technique described in Clabes results in tips having dimensional limitations. For example, in order to achieve the desired levels of accuracy in measurements of trench critical dimensions, it is necessary to manufacture AFM tips in which the points can be manufactured to have a length of less than 1,000 .ANG., with the tips having an overall maximum diameter of less than 2,000 .ANG., for a trench having a depth of 0.25 .mu.m. It is difficult to control the known techniques of using CVD to form three-point tips so as to obtain points having the desired dimensions. Further, the organic-metallic nature of the three-point tips made by the known techniques using CVD is less desirable than three-point tips manufactured of substantially pure silicon.